Net-Zero Energy Homes

While a “net-zero-energy” home sounds like it belongs in a sci-fi movie, such homes are actually part of our present-day reality. There are homes that generate all the power they need, and they don’t look like space stations. And it may become a mandate in the not-too-distant future; in California, which has the most aggressive energy-efficiency requirements in the U.S., the goal is for all new residential construction to be net-zero by 2020. 

While it’s very feasible to build a new net-zero home or retrofit your existing home, it’s also definitely not as simple as putting a giant solar array on your home and calling it a day. Here’s what you need to know if you’re inspired to get rid of your house’s carbon footprint.

Not unlike religious factions, net-zero supporters argue about how “pure” a house has to be before it qualifies. For instance, should all the energy that is used in building the house be part of the equation? The International Living Future Institute defines a net-zero home as one where “one hundred percent of the building’s energy needs are supplied by onsite renewable energy on a net annual basis,” and doesn’t use any combustible energy (including natural gas). Other building-policy think tanks, including the National Renewable Energy Laboratory, have come up with a range of definitions that include homes where renewable energy comes from off-site sources or is used to offset a certain amount of fossil fuel use. It remains to be seen what the requirements will be when the well-known LEED green-building program decides to incorporate net-zero. If you are interested in building a home through a formal net-zero certification program, currently the only option is the aforementioned International Living Future Institute’s Net-Zero-Energy Building Certification--a rigorous process that involves submitting a year’s worth of utility bills after you’ve moved in. 

The good news is that net-zero doesn’t have to cost the moon. According to a study commissioned by the California Zero Net Buildings initiative, it adds only $2 to $8 per square foot. “Custom home builders who are developing [zero-net-energy] homes right now indicate that there are nominal additional costs and that the key issue to achieve ZNE is design and quality construction,” states the website. 

Regardless of the exact definition, the path to achieving net-zero is pretty clear. The strategy is to reduce a home’s energy use as much as possible, and then use renewable energy for what it does need. “Conservation comes first—that’s the hard part,” says Allen Gilliland of One Sky Homes, a small custom builder who has built or retrofitted four net-zero homes in the San Francisco Bay Area.

What uses the most energy within a home? According to 2010 data collected by the U.S. Department of Energy, the HVAC system accounts for 43 percent. Hot water heaters account for 13 percent, and lighting is another 10 percent.

Since roughly half the energy used in a home is for heating and cooling, the first step is the standard architectural practice of designing a house for a particular climate: using overhangs, appropriately-sized windows, and other passive design features to mitigate extreme temperatures naturally. Then to get to net-zero, some homes follow Passive House guidelines to create a high-tech, tightly sealed and insulated building envelope. Passive House design uses an HRV [heat recovery ventilator] to extract heat that you produce in the course of daily household activities (cooking, taking a shower) and then uses that heat to keep the house warm instead of exhausting it away. The result is a more comfortable house that uses up to 90 percent less energy for heating and cooling. Not surprisingly, the Passive House Institute is about to launch new certification programs for Passive House Plus (net-zero) and Passive House Premium (net-positive).

In addition to heating and cooling, though, there are all the other ways a home uses energy to take into account. “The two elephants in the room are the energy required to heat water and plug loads [everything that uses electricity],” says Gilliland. To make sure there’s enough water for hot showers, a common solution is a solar hot-water system with a backup gas heater or electric heat pump. Using only energy-efficient electric appliances, including an induction cooktop, simplifies the complexity of the puzzle and minimizes the use of fossil fuels. “You make the house into as much of an electrical machine as possible,” says Houseplans.com architect Cathy Schwabe, based in Oakland, California, who has designed a near-net-zero house. 

A house with a single broad roof instead of multiple roofs, like this design by architect Cathy Schwabe, is well-suited for solar panels. Plan 891-1

Generating your own energy

One wrinkle of net-zero building is having to figure how much electricity the house will use—a prediction that takes some finesse—in order to figure out how much energy you need to generate. “Modeling this information is tricky and is an additional cost, but the knowledge should pay for itself,” says Schwabe.

The Cottle zero-energy home in San Jose, designed and built by One Sky Homes, is LEED Platinum and Passive House certified, as well as net-zero rated through California’s HERS (Home Energy Rating System). 

Built to Passive House standards, the Cottle house reduced its energy usage to 25 percent of what a typical house of a similar size uses.

For the 3,200-square-foot net-zero home he built in San Jose, Gilliland installed an array of 30 panels that produces 6.2kW. In this case, Gilliland purposely added extra capacity so the homeowner would have surplus energy for his electric vehicle—the equivalent of about 15,000 miles per year.  
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